1. Field of the Invention
The present invention relates to torsional damper type flywheel devices having viscous damping mechanisms to suppress resonance.
2. Description of the Related Art
Flywheel devices which include drive side and driven side flywheels, a single kind of spring mechanism for connecting the drive side and driven side flywheels, and a friction mechanism are known, for example, in U.S. Pat. Nos. 4,468,207, 4,274,524, 4,351,168, 2,042,570, 4,445,876, 2,729,079, 2,437,537, 4,663,983, 4,220,233, and 4,002,043; GB-A-2,000,257; DE-A-2,926,012; Automotive Engineering, vol. 93, page 85; Japanese Utility Model Publications SHO 61-23542, SHO 61-23543, SHO 61-23544, SHO 59-113548, SHO 59-108848, SHO 56-6676, and SHO 56-109635; and Japanese Patent Publications SHO 61-59040, SHO 61-59042, and SHO 61-52423.
Any one of the above-described prior art flywheel devices has a single vibrational characteristic defined by the spring mechanism which may include a plurality of coil springs arranged in series with or parallel to each other. Due to the vibrational characteristic, the flywheel device has a single first mode resonance speed throughout the entire range of engine speeds. The single resonance speed is usually set lower than the idling speed of the engine, and when the engine speed passes through the resonance speed during start-up or stopping of the engine, the torsional vibration of the flywheel is amplified. To suppress the amplification, a continuously sliding friction mechanism (often called a hysteresis mechanism) which continuously slides throughout the entire range of engine speeds is usually disposed between the drive side and driven side flywheels.
However, there are two problems with the above-described prior art flywheel devices. One problem is that a considerably large resonance occurs at the resonance speed, in spite of the provision of the friction mechanism, because the characteristic of the flywheel device is determined more by the spring mechanism than by the friction mechanism. The other problem is that the continuously sliding friction mechanism degrades the acceleration transmittance rate (which corresponds to a speed variation or torque variation absorbing effect of the divisional type flywheel device) at the standard range of engine speeds above the idling speed. This is because a temporary sticking often occurs in the friction mechanism and because the frictional force makes the flywheel device operate as if the flywheel device were a non-divisional type device.
The U.S. patent application, Ser. No. 07/093,573, filed Sep. 4, 1987 (corresponding to European Patent Application No. 87307821.6), presents a quite different type of flywheel device designed to overcome the problems of the above-described prior art flywheel devices. That flywheel device includes drive side and driven side flywheels, two kinds of spring mechanisms (called a K spring mechanism and a K1 spring mechanism) arranged parallel to each other between the drive side and driven side flywheels, and a momentarily sliding friction mechanism arranged in series with the K1 spring mechanism. The vibrational behavior of that flywheel device can be understood through reference to the Shock and Vibration Hand Book, vol. 2, McGraw Hill, though it does not relate to a flywheel device.
More particularly, that flywheel device has two vibrational characteristics shown by the two-dotted lines in FIG. 5, namely, a K characteristic where only the K spring mechanism operates without being accompanied by the sliding of the friction mechanism and a K+K1 characteristic where both the K and K1 spring mechanisms operate with the sliding of the friction mechanism. In the standard range of engine speeds, the flywheel device operates according to the K+K1 characteristic, because no excessively great torque usually acts at the standard range of engine speeds. Because the friction mechanism does not slide at that time, the speed variation absorbing effect is greatly improved. When the engine speed approaches the resonance speed of the K+K1 characteristic at start-up or stopping of the engine, the torsional angle between the drive side and driven side flywheels increases. Thus, the K1 spring mechanism is more compressed, and the torque acting on the friction mechanism increases to finally cause the friction mechanism to slide. Upon sliding of the friction spring mechanism, the flywheel device changes its vibrational behavior from the K+K1 characteristic to the K characteristic by jumping over the resonance speed of the K+K1 characteristic. After jumping, when the rotational speed changes away from the resonance speed of the K+K1 characteristic, the torsional angle between the drive side and driven side flywheels gradually decreases, and the sliding of the friction mechanism finally stops. Then, the flywheel device again operates according to the K+K1 characteristic. In this way, occurrence of resonance while the rotational speed of the flywheel device passes through the resonance speed of the K+K1 characteristic is prevented.
However, there are some problems to be solved in the above-described flywheel device having the momentarily sliding friction mechanism. Namely, the slide beginning or stopping point of the momentarily sliding friction mechanism may fluctuate due to a manufacturing tolerance of the friction mechanism and the abrasion of the abrasive member of the friction mechanism during use. Though it may be effective to provide some device to cause the friction mechanism to begin or stop sliding at a predetermined torsional angle, such a device would make the flywheel device large in size.